There is conventionally known an image forming apparatus including a cartridge support unit configured to support a plurality of cartridges and to be mounted in and withdrawn from a main casing. Further, the image forming apparatus is so configured as to prevent the cartridge support unit from being forcibly mounted in the main casing.
More specifically, when the cartridge support unit moves in a mounting direction toward a mounted position inside the main casing, the cartridge support unit is configured to receive a resistance force in a direction opposite to the mounting direction from a resistance force applying portion provided in the main casing by abutting on the resistance force applying portion. The resistance force applied to the cartridge support unit by the resistance force applying portion at this time gradually increases, and is then turned into a force pressing the cartridge support unit in the mounting direction.